For over a decade it has been known that breast cancer subtypes are dictated by their expressed genomes or transcriptomes and that these expression profiles dictate tumor aggressiveness and patient outcomes. Despite this conceptual breakthrough, the creation of targeting approaches to alter these transcriptomes has not yet been feasible due to our limited understanding of the factors that activate and maintain gene expression in these tumors. This proposal focuses on Bromodomain and Extra Terminal (BET) proteins, which are readers of the histone code, and their role in regulating breast cancer growth and progression. We will focus on triple negative breast cancers (TNBC) which lack estrogen, progesterone, and HER2 receptors because patients with these tumors do not reap the benefits of targeted therapies directed to these receptors. Identifying the mechanisms that control the expressed genomes of these tumors should reveal new avenues for ameliorating this relatively untreatable disease. Our preliminary studies have revealed that loss of BET protein function results in extensive suppression of genes involved in mitosis and cytokinesis and causes polyploidy in TNBC cells. This defect is followed either by apoptosis or senescence in numerous cell lines. Furthermore, the BET inhibitor, JQ1, displays significant efficacy and inhibits the growth of tumors in three different xenograft models. We will expand upon these foundational studies to uncover the mechanisms by which suppression of BET proteins inhibits growth and promotes death of TNBC cells. In Aim 1, we will identify the BET proteins and their transcriptional targets that maintain normal ploidy in TNBC cell lines as well as determine if FOXM1 is a major intermediate in BET control of normal cell cycle progression. Aim 2 will focus on deciphering the specific defects induced by BET inhibitors that lead to polyploidy in TNBC cells and assessing whether these can be leveraged to enhance sensitivity to chemotherapies that also target mitosis. Lastly, Aim 3 will identify the specific mechanisms that dictate the decision to apoptose or senesce in response to the loss of BET function in TNBC cells, with the goal of ensuring an apoptotic cell fate in response to BET inhibitors. Together, these studies will provide key mechanistic insights into the roles of BET proteins in TNBC and reveal approaches for enhancing their activity in this disease. These studies should lead to the development of a new therapeutic approach that globally targets the transcriptome of aggressive breast cancers with the goal of improving patient outcomes.